James dns server does not properly cache results when a CNAME is used for the domain (RFC-2821 sections 3.6 and 5)
mail.domain.com. IN CNAME domain.com
domain.com. IN MX 0 mx0.domain.com
mx0.domain.com. IN A XXX.XXX.XXX.XXX
lookup MX addresses for "mail.domain.com" multiple times will result always in real lookup because "cache.isSuccesfull" return false when the result is a CNAME.